For many years, the blood brain barrier has been considered a major obstacle to systemic therapy to reach the central nervous system for lysosomal storage diseases. However, recent data have indicated that high serum levels of beta-glucuronidase can alter the central nervous system lesions and behavioral abnormalities in adult mucopolysaccharidosis VII mice. But the question remains, is this observation limited to mice and to mucopolysaccharidosis VII or will it be true for large animals as models for children and for other lysosomal storage diseases? This grant proposal is designed to answer these questions. We have shown improvement in central nervous system neuropathological lesions in mucopolysaccharidosis I and VII dogs with constant high serum levels of alpha-L-iduronidase and beta-glucuronidase, respectively, following neonatal, intravenous retroviral gene therapy. However, because the mucopolysaccharidosis dogs lack clinical signs of the central nervous system lesions, improvement in neurological function in the large animals could not be evaluated. This grant application proposes to test the levels of serum activity associated with transit across the blood brain barrier using somatic (liver-based) gene therapy in cats with alpha-mannosidosis. Alpha-mannosidosis cats have significant, well-documented neurological signs of disease, with death by six months of age if untreated, and well-described neuropathological lesions. Alpha-mannosidosis cats have also been shown to respond to bone marrow transplantation and direct brain injection of a viral vector so it will be clear if high serum enzyme activity is successful. Thus, we propose to determine if high constant serum alpha- mannosidase activity will cross the blood brain barrier and abrogate the clinical and neuropathological disease. This is an important proof of principle before proposing gene therapy clinical trials to produce high serum enzyme activity in any of the 60% of lysosomal storage diseases with central nervous system lesions found in children. PUBLIC HEALTH RELEVANCE: This grant proposes to treat young cats with the naturally occurring genetic storage disease, alpha-mannosidosis, by using gene therapy. The goal is to produce enough normal therapeutic enzyme in the liver to allow it to cross the blood brain barrier and prevent or reverse the disease in the brain.